Dino digestion in a test tube

Copyright Frank Luerweg, University of Bonn

Jürgen Hummel and his colleagues mimicked dinosaur digestion in the laboratory using gut bacteria from sheep.

Schoolchildren and scientists alike often wonder how the plant-eating dinosaurs of the Jurassic got to be so big. For scientists, the question is intriguing because these giants, known as sauropods, lived before the arrival of flowering plants, the staple of the modern herbivore’s diet. The only vegetarian options available at the time were an assortment of ferns, conifers and other non-flowering plants — plants that modern animals generally don’t touch and scientists don’t think are very nutritious. But by re-creating dinosaur “stomachs” in the lab, researchers discovered that these plants may be getting a bad rap.

“It’s generally thought that these food plants must have been very low quality,” says Jürgen Hummel, a zoologist at the University of Bonn in Germany who led the new research. But he says the energy content of many of these plants turned out to be “surprisingly high.”

These findings cast doubt on a common explanation of why sauropods were the largest land animals ever to walk the planet. Some scientists think sauropods had to be so big because the quality of their diets was so poor: Larger animals have larger digestive tracts and that means that food stays in their guts longer. This longer gut retention time leads to better digestion because plant fiber is digested very slowly. But if non-flowering plants are nearly as nutritious as flowering plants, then this doesn’t explain why sauropods were so much bigger than today’s herbivores.

To better understand sauropod dining options, Hummel and his colleagues used a technique that’s frequently used to assess the quality of livestock feed. They took modern relatives of ancient ferns, conifers and other non-flowering plants, and dried and ground them up. Next, they placed each plant into a different glass syringe containing bacteria from a sheep’s gut. Nearly all modern herbivores rely on bacteria to help break down the tough, fibrous parts of plants, so Hummel and his colleagues reasoned sauropods probably did too.

After incubating the syringes, Hummel and his colleagues regularly checked on the tubes to measure the amount of gas produced by the bacteria during fermentation: More gas in the tube meant the plants had more energy available to fuel the bacteria’s fermentation. Some non-flowering plants lived up to their bad reputation, but others, including horsetails, ginkgo, certain conifers and some ferns, had nearly as much energy as the modern grasses and herbaceous flowering plants that were tested.

“The central point is that the quality, in terms of energy content, is not as bad as often expected,” says Hummel, who along with his colleagues reported these results online Feb. 5 in the Proceedings of the Royal Society B.

Jeremy Midgley, head of the botany department at the University of Cape Town in South Africa, who was not involved in the study, doesn’t think these results are incompatible with the poor-quality food explanation. “Nitrogen concentration is the nutrient which most generally affects herbivore feeding ecology,” he says, not energy content, because nitrogen is needed to make proteins. The study's reported protein contents (a good indicator of nitrogen) of the non-flowering and flowering plants support the traditional view that non-flowering plants are less nutritious in this respect, he says. Hummel, however, notes that energy content is very important to herbivore feeding ecology and that some studies suggest that low protein availability in the environment can lead to the evolution of low protein requirements in animals.

Regardless of the interpretation of the results, Midgley says, “it’s great that someone has taken the hypothesis on.” Bruce Tiffney of the University of California at Santa Barbara, who has studied the relationship between plants and dinosaurs, agrees. “They’ve done a very nice job of testing hypotheses,” he says. “This paper will make me hone my arguments more closely.”